[Mesa-stable] libatomic fixes for 18.1 stable

[Matt Turner]

Hi Dylan,

In the continuing struggle to use 64-bit atomics, please cherry-pick the
following patches to the 18.1 stable branch:

54ba73ef102f7b9085922686bb31719539e0dc3c (configure.ac/meson.build: Fix -latomic test)
54bbe600ec2654bcf43c3378e84d972e0a7ce4b6 (configure.ac: rework -latomic check)
b29b5a82a17e845e73ebef845176d9d1f44a2d00 (meson: Fix -latomic check)
9d547a7617fbcbc29f75ed85ff774807ddca5ce0 (meson: Test for __atomic_add_fetch in atomic checks)
baf16b2ea3a72d01712d4e16fae97606ac68dff0 (configure.ac: Test for __atomic_add_fetch in atomic checks)

I hope I never have to see this code again.

Thanks,
Matt
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